NateKendrick
Junior Member
One thing I've noticed in a lot of advice columns and articles is that most discussions of power draw are based on bench testing. Hook up the battery, ESC, motor, and prop, with a watt-meter in-line. Run it up, and check the power draw. Put it on a force balance and check the thrust.
This makes sense for quad-rotors, but in-flight, with forward speed, the power draw and thrust of an airplane prop will be much less than static.
http://www.apcprop.com/v/downloads/PERFILES_WEB/datalist.asp
For example, from the APC web site, an APC 9x6E has the following performance data (condensed):
PROP RPM = 8000
V__ J___ Pe____ Ct____ Cp___ PWR_ Torque Thrust
(mph) (Adv Ratio) ____________(Hp)_ (In-Lbf) (Lbf)
02.1 0.03 0.0482 0.1006 0.0634 0.154 1.214 1.346
10.4 0.15 0.2251 0.0814 0.0549 0.134 1.052 1.089
20.7 0.30 0.4125 0.0644 0.0474 0.115 0.909 0.862
31.1 0.46 0.5614 0.0501 0.0407 0.099 0.779 0.670
39.3 0.58 0.6494 0.0374 0.0332 0.081 0.636 0.500
49.7 0.73 0.6884 0.0196 0.0208 0.050 0.398 0.262
60.1 0.88 -0.0003 0.0000 0.0053 0.013 0.101 0.000
Note that static it draws .154 HP (115 W) but at 40 mph it drops to 0.081 HP (60 W). This would mean about half the current draw. Thrust has dropped from 1.3 lbs to only .5 lbs. This plan will start like a rocket and climb fast, but have low top end speed.
An APC 8x8E on the other hand:
PROP RPM = 8000
V__ J___ Pe____ Ct____ Cp___ PWR_ Torque Thrust
(mph) (Adv Ratio) ____________(Hp)_ (In-Lbf) (Lbf)
02.6 0.04 0.0580 0.0958 0.0714 0.096 0.760 0.800
10.5 0.17 0.2157 0.0997 0.0799 0.108 0.849 0.832
21.0 0.35 0.4012 0.1031 0.0888 0.120 0.944 0.861
31.4 0.52 0.5503 0.0945 0.0891 0.120 0.947 0.789
39.3 0.65 0.6411 0.0813 0.0822 0.111 0.874 0.679
49.8 0.82 0.7353 0.0611 0.0682 0.092 0.725 0.510
60.2 0.99 0.7998 0.0385 0.0478 0.065 0.509 0.321
70.7 1.17 0.7669 0.0134 0.0204 0.027 0.217 0.112
Notice how the power draw is low at 2 mph (it's stalled), and peaks between 20 and 30 mph. At 40 mph, it has roughly the same efficiency (Pe), but it both draws more power (83 W) and provides more thrust (.68 lbs). At 50 mph it produces twice the thrust of the 9x6E.
The interesting thing to me is, say you're sizing your system, and you want to make sure you don't pull over your amp ratings. If you bench test, it might show too much. But unless you're doing 3D, you'll never see that kind of draw except for the split second you when you take off. Once the aircraft is moving, current load drops off quickly. You could conceivably use a lower spec (lighter) motor and never risk your system. And calculated flight times, even at full throttle, should be better. It's all well and good if you want safety margin, but if you want to maximize performance, it seems there is a lot of room for trimming specs.
This may be old news to old hands, but I wanted to open the discussion and see what I could learn. Any tips from more experienced builders?
This makes sense for quad-rotors, but in-flight, with forward speed, the power draw and thrust of an airplane prop will be much less than static.
http://www.apcprop.com/v/downloads/PERFILES_WEB/datalist.asp
For example, from the APC web site, an APC 9x6E has the following performance data (condensed):
PROP RPM = 8000
V__ J___ Pe____ Ct____ Cp___ PWR_ Torque Thrust
(mph) (Adv Ratio) ____________(Hp)_ (In-Lbf) (Lbf)
02.1 0.03 0.0482 0.1006 0.0634 0.154 1.214 1.346
10.4 0.15 0.2251 0.0814 0.0549 0.134 1.052 1.089
20.7 0.30 0.4125 0.0644 0.0474 0.115 0.909 0.862
31.1 0.46 0.5614 0.0501 0.0407 0.099 0.779 0.670
39.3 0.58 0.6494 0.0374 0.0332 0.081 0.636 0.500
49.7 0.73 0.6884 0.0196 0.0208 0.050 0.398 0.262
60.1 0.88 -0.0003 0.0000 0.0053 0.013 0.101 0.000
Note that static it draws .154 HP (115 W) but at 40 mph it drops to 0.081 HP (60 W). This would mean about half the current draw. Thrust has dropped from 1.3 lbs to only .5 lbs. This plan will start like a rocket and climb fast, but have low top end speed.
An APC 8x8E on the other hand:
PROP RPM = 8000
V__ J___ Pe____ Ct____ Cp___ PWR_ Torque Thrust
(mph) (Adv Ratio) ____________(Hp)_ (In-Lbf) (Lbf)
02.6 0.04 0.0580 0.0958 0.0714 0.096 0.760 0.800
10.5 0.17 0.2157 0.0997 0.0799 0.108 0.849 0.832
21.0 0.35 0.4012 0.1031 0.0888 0.120 0.944 0.861
31.4 0.52 0.5503 0.0945 0.0891 0.120 0.947 0.789
39.3 0.65 0.6411 0.0813 0.0822 0.111 0.874 0.679
49.8 0.82 0.7353 0.0611 0.0682 0.092 0.725 0.510
60.2 0.99 0.7998 0.0385 0.0478 0.065 0.509 0.321
70.7 1.17 0.7669 0.0134 0.0204 0.027 0.217 0.112
Notice how the power draw is low at 2 mph (it's stalled), and peaks between 20 and 30 mph. At 40 mph, it has roughly the same efficiency (Pe), but it both draws more power (83 W) and provides more thrust (.68 lbs). At 50 mph it produces twice the thrust of the 9x6E.
The interesting thing to me is, say you're sizing your system, and you want to make sure you don't pull over your amp ratings. If you bench test, it might show too much. But unless you're doing 3D, you'll never see that kind of draw except for the split second you when you take off. Once the aircraft is moving, current load drops off quickly. You could conceivably use a lower spec (lighter) motor and never risk your system. And calculated flight times, even at full throttle, should be better. It's all well and good if you want safety margin, but if you want to maximize performance, it seems there is a lot of room for trimming specs.
This may be old news to old hands, but I wanted to open the discussion and see what I could learn. Any tips from more experienced builders?
Last edited: